This invention relates generally to digital amplifiers and, in particular, to the use of analog feedback to linearize such amplifiers.
As digital technology advances, analog sources are diminishing in favor of digital data streams. To effect amplification, early approaches using digital-to-analog converters and analog amplification are evolving to approaches providing high power directly from digital data streams, known as fully digital amplifiers.
The ultimate output of such amplifiers, however, is an analog signal. Linearization techniques using feedback from this output therefore require analog-to-digital conversion in order to interact effectively with the incoming digital data stream. The components required to implement the linearization are just as expensive as the digital-to-analog converters obviated by fully digital designs.
The need therefore exists for a method by which operation of a digital system can be altered using an analog source.
The present invention provides a technique whereby an analog voltage is used to directly alter the timebase of a pulse-width converter, thus modifying a digitally-generated system output with an analog source.
The input to the amplifier is coupled to a first counter clocked by a voltage-controlled oscillator (VCO), and second counter driven by a fixed clock. The output of the first counter is ultimately delivered to a load device following appropriate filtering. The non-inverting input of a differential amplifier is coupled to the load device, and the inverting input is coupled to the output of the second counter.
The output of the differential amplifier forms the control voltage input to the VCO, which increases output frequency in response to increasing input voltage. An increase in frequency at the output of the VCO ultimately reduces the load voltage, thereby linearizing an otherwise digital amplifier with an analog signal.